Turbine performance depends on available energy content per unit of drive gas and the blade tangential velocity, U, wherein the available energy for the turbine pressure ratio may be expressed as an ideal velocity, C. The turbine velocity ratio or blade-jet-speed ratio, U/C, may be used to empirically characterize the available energy and blade tangential velocity with respect to turbine efficiency. The blade-jet-speed ratio may also be defined as the ratio of circumferential speed and the jet velocity corresponding to an ideal expansion from inlet total to exit total conditions.
Turbochargers often operate at conditions with low blade-jet-speed ratio values (e.g., U/C<0.7). Radially stacked turbine rotors typically have an optimum U/C value of 0.7 where they achieve their highest efficiency. This rotor characteristic reduces the efficiency of the turbines at low blade-jet-speed ratio conditions. Further, the inducer of a radially stacked turbine rotor has a blade (metal) angle of zero degrees at its leading edge, which leads to positive incidence (flow angle minus blade angle) in the inducer when the U/C value drops below 0.7. The positive incidence can cause flow separation in the rotor with reduction in turbine efficiency.
A need exists for blades that reduce positive incidence at low U/C values. Various exemplary methods, devices, systems, etc., disclosed herein aim to meet this need and/or other needs.